diff --git a/CHANGELOG.md b/CHANGELOG.md
index fcb0cf8c..2bbcee6a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed issue where single quotes could not be used in search queries. [#629](https://github.com/sourcebot-dev/sourcebot/pull/629)
 - Fixed issue where files with special characters would fail to load. [#636](https://github.com/sourcebot-dev/sourcebot/issues/636)
 - Fixed Ask performance issues. [#632](https://github.com/sourcebot-dev/sourcebot/pull/632)
+- Fixed regression where creating a new Ask thread when unauthenticated would result in a 404. [#641](https://github.com/sourcebot-dev/sourcebot/pull/641)
 
 ## [4.10.0] - 2025-11-24
 
diff --git a/packages/db/prisma/migrations/20251129063148_change_chat_created_by_to_optional/migration.sql b/packages/db/prisma/migrations/20251129063148_change_chat_created_by_to_optional/migration.sql
new file mode 100644
index 00000000..d0fd2ba0
--- /dev/null
+++ b/packages/db/prisma/migrations/20251129063148_change_chat_created_by_to_optional/migration.sql
@@ -0,0 +1,5 @@
+-- First, remove the NOT NULL constraint on the createdById column.
+ALTER TABLE "Chat" ALTER COLUMN "createdById" DROP NOT NULL;
+
+-- Then, set all chats created by the guest user (id: 1) to have a NULL createdById.
+UPDATE "Chat" SET "createdById" = NULL WHERE "createdById" = '1';
diff --git a/packages/db/prisma/schema.prisma b/packages/db/prisma/schema.prisma
index 2e87ad4f..95460852 100644
--- a/packages/db/prisma/schema.prisma
+++ b/packages/db/prisma/schema.prisma
@@ -437,8 +437,8 @@ model Chat {
 
   name String?
 
-  createdBy   User   @relation(fields: [createdById], references: [id], onDelete: Cascade)
-  createdById String
+  createdBy   User?   @relation(fields: [createdById], references: [id], onDelete: Cascade)
+  createdById String?
 
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
diff --git a/packages/web/src/features/chat/actions.ts b/packages/web/src/features/chat/actions.ts
index a987dbbc..a9053bd2 100644
--- a/packages/web/src/features/chat/actions.ts
+++ b/packages/web/src/features/chat/actions.ts
@@ -1,7 +1,6 @@
 'use server';
 
 import { sew } from "@/actions";
-import { SOURCEBOT_GUEST_USER_ID } from "@/lib/constants";
 import { ErrorCode } from "@/lib/errorCodes";
 import { chatIsReadonly, notFound, ServiceError, serviceErrorResponse } from "@/lib/serviceError";
 import { createAmazonBedrock } from '@ai-sdk/amazon-bedrock';
@@ -34,13 +33,13 @@ const logger = createLogger('chat-actions');
 
 export const createChat = async () => sew(() =>
     withOptionalAuthV2(async ({ org, user, prisma }) => {
-        const isGuestUser = user?.id === SOURCEBOT_GUEST_USER_ID;
+        const isGuestUser = user === undefined;
 
         const chat = await prisma.chat.create({
             data: {
                 orgId: org.id,
                 messages: [] as unknown as Prisma.InputJsonValue,
-                createdById: user?.id ?? SOURCEBOT_GUEST_USER_ID,
+                createdById: user?.id,
                 visibility: isGuestUser ? ChatVisibility.PUBLIC : ChatVisibility.PRIVATE,
             },
         });